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                <h4 id="认识HashMap"><a href="#认识HashMap" class="headerlink" title="认识HashMap"></a>认识HashMap</h4><blockquote>
<p>首先了解一下HashMap<br><a target="_blank" rel="noopener" href="https://zhuanlan.zhihu.com/p/21673805">https://zhuanlan.zhihu.com/p/21673805</a></p>
<p>顺便附两个《小灰漫画》，感觉讲的不错：</p>
<ul>
<li><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzIxMjE5MTE1Nw==&mid=2653191907&idx=1&sn=876860c5a9a6710ead5dd8de37403ffc&chksm=8c990c39bbee852f71c9dfc587fd70d10b0eab1cca17123c0a68bf1e16d46d71717712b91509&scene=21#wechat_redirect">什么是HashMap</a></li>
<li><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s?__biz=MzIxMjE5MTE1Nw==&mid=2653192000&idx=1&sn=118cee6d1c67e7b8e4f762af3e61643e&chksm=8c990d9abbee848c739aeaf25893ae4382eca90642f65fc9b8eb76d58d6e7adebe65da03f80d&scene=21#wechat_redirect">多线程下的HashMap</a></li>
</ul>
</blockquote>
<h5 id="Map接口四个常用实现类"><a href="#Map接口四个常用实现类" class="headerlink" title="Map接口四个常用实现类"></a>Map接口四个常用实现类</h5><p>Java为数据结构中的映射定义了一个接口<code>java.util.Map</code>，此接口主要有四个常用的实现类：</p>
<ul>
<li>HashMap</li>
<li>Hashtable</li>
<li>LinkedHashMap</li>
<li>TreeMap</li>
</ul>
<p>类继承关系如下图所示：</p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/26341ef9fe5caf66ba0b7c40bba264a5_r.jpg" alt="Map继承关系"></p>
<h5 id="HashMap"><a href="#HashMap" class="headerlink" title="HashMap"></a>HashMap</h5><p>根据键的 hashCode 存储数据。</p>
<p>访问很快，遍历顺序不确定。</p>
<p>键允许一个为null的，记录允许多个为null的。</p>
<p>==非线程安全，即任一时刻可以有<strong>多个线程同时写HashMap</strong>，可能会导致数据的不一致==</p>
<p>如果需要满足线程安全，</p>
<ul>
<li>可以用 <strong><code>Collections.synchronizedMap()</code>方法</strong>使HashMap具有线程安全的能力</li>
<li>或者使用**<code>ConcurrentHashMap</code>**类</li>
</ul>
<h5 id="Hashtable"><a href="#Hashtable" class="headerlink" title="Hashtable"></a>Hashtable</h5><p><strong>遗留类，不建议使用</strong>。</p>
<p>与<code>HashMap</code>类似，不过是继承的<code>Dirctoinary</code>类。</p>
<ul>
<li>不需要线程安全的场合，用<code>HashMap</code>替代；</li>
<li>需要线程安全的场合，用<code>ConcurrentHashMap</code>替代</li>
</ul>
<p><code>Hashtable</code>是线程安全的，但是并发性不如<code>ConcurrentHashMap</code>，因为<code>ConcurrentHashMap</code>引入了分段锁。</p>
<h5 id="LinkedHashMap"><a href="#LinkedHashMap" class="headerlink" title="LinkedHashMap"></a>LinkedHashMap</h5><p><code>HashMap</code>的子类，带顺序的<code>HashMap</code>，<strong>保留了插入顺序</strong>。</p>
<p><code>Iterator</code>遍历时候，先得到的记录肯定是先插入的。</p>
<h5 id="TreeMap"><a href="#TreeMap" class="headerlink" title="TreeMap"></a>TreeMap</h5><p>实现了<code>SortedMap</code>接口，<strong>按key排序</strong>，默认按key的升序排序。</p>
<p><code>Iterator</code>遍历时候，是按排过序的key遍历的。</p>
<h4 id="HashMap原理"><a href="#HashMap原理" class="headerlink" title="HashMap原理"></a>HashMap原理</h4><p>搞清楚HashMap，首先需要知道HashMap是什么，即它的<strong>存储结构-字段</strong>；其次弄明白它能干什么，即它的<strong>功能实现-方法</strong>。下面我们针对这两个方面详细展开讲解。</p>
<h5 id="存储结构-字段"><a href="#存储结构-字段" class="headerlink" title="存储结构-字段"></a>存储结构-字段</h5><p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/image-20210421191347092.png" alt="image-20210421191347092"></p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/v2-06aa0a5efbb324a67b23029206d9d9ac_1440w.jpg" alt="HashMap存储结构"></p>
<p>每一个<strong>下标</strong>位置被称为==桶==</p>
<ul>
<li>JDK1.8之前：数组 + 链表</li>
<li>JDK1.8之后：数组 + 链表 + 红黑树<ul>
<li>链表长度大于8，转换为红黑树</li>
</ul>
</li>
</ul>
<p>哈希表解决冲突方法：</p>
<ul>
<li>开放地址法</li>
<li>链地址法</li>
</ul>
<p>JDK中就是用的链地址法。简单来说，就是数组加链表的结合。</p>
<h5 id="功能实现-方法"><a href="#功能实现-方法" class="headerlink" title="功能实现-方法"></a>功能实现-方法</h5><p>一个Hash存储的 过程</p>
<blockquote>
<p>map.put(“美团”,”小美”);</p>
</blockquote>
<p>首先使用hashcode()方法，对key得到其HashCode值，然后Hash算法的后两步（高位运算和取模）定位该key对应的存储位置索引值。</p>
<p>可能定位到相同位置，就是发生了<strong>Hash碰撞</strong>。</p>
<ul>
<li>Hash算法越好，越不容易发生哈希碰撞。</li>
<li>当然如果Hash桶足够大，即使差的Hash算法也不容易碰撞。</li>
</ul>
<p>所以说要在<strong>空间（Hash桶）</strong>和<strong>时间（Hash散列算法）</strong>成本之间进行衡量。</p>
<p>==要做到碰撞概率小且空间少，好的Hash算法和扩容机制。==</p>
<h6 id="扩容"><a href="#扩容" class="headerlink" title="扩容"></a>扩容</h6><p>如何扩容：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">int</span> threshold<span class="token punctuation">;</span>             <span class="token comment">// 所能容纳的key-value对极限 </span>
<span class="token keyword">final</span> <span class="token keyword">float</span> loadFactor<span class="token punctuation">;</span>    <span class="token comment">// 负载因子</span>
<span class="token keyword">int</span> modCount<span class="token punctuation">;</span>  	<span class="token comment">// hashmap内部结构变化的次数(不包含key对应value被覆盖的情况)</span>
<span class="token keyword">int</span> size<span class="token punctuation">;</span>	<span class="token comment">// 实际存储的数量</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span></span></code></pre>

<ul>
<li><code>Node[] table</code> 的默认length为16</li>
<li><code>loadFactor</code> ，负载因子，默认为0.75（一般不要改）<ul>
<li>当空间不大，不要求时间的时候：<code>loadFactor</code> 可以增大，可以设为大于1</li>
<li>当空间足够，要求时间效率高，<code>loadFactor</code> 可以适当减小</li>
</ul>
</li>
<li>threshold = length * Load factor</li>
<li>当超出threshold时候，就进行扩容</li>
</ul>
<p>==Hash算法本质上就是三步：取key的hashCode值、高位运算、取模运算==</p>
<h6 id="JDK1-7扩容"><a href="#JDK1-7扩容" class="headerlink" title="JDK1.7扩容"></a>JDK1.7扩容</h6><p>扩容机制使用JDK1.7版本的来解释一下，因为它比较简单：</p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/e5aa99e811d1814e010afa7779b759d4_r.jpg" alt="扩容"></p>
<p>这里是使用key % size，来确定放入哪个哈希桶的。比如第一个中，3%2=1，所以说放到索引为1的桶中。</p>
<p>这里假设<code>LoadFactor = 1</code> , 那么<strong>当k-v的size大于桶的size的时候</strong>就扩容。所以说当<code>k_v.size() = 3</code>的时候，进行扩容，边为原来的<strong>两倍</strong>。</p>
<p><strong>JDK1.7中扩容之后需要重新Hash</strong>，即1.2.3的过程。</p>
<h6 id="JDK1-8扩容"><a href="#JDK1-8扩容" class="headerlink" title="JDK1.8扩容"></a>JDK1.8扩容</h6><p><strong>JDK1.8改进了，不需要再ReHash</strong>。（下面这一块关于位运算的没能看太懂）</p>
<p>使用了<strong>位运算</strong>的特点：<br>元素在重新计算hash之后，因为n变为2倍，那么n-1的mask范围在高位多1bit(红色)，因此新的index就会发生这样的变化：</p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/b2cb057773e3d67976c535d6ef547d51_r.jpg" alt="位运算"></p>
<p>因此，我们在扩充HashMap的时候，不需要像JDK1.7的实现那样重新计算hash，<strong>只需要看看原来的hash值新增的那个bit是1还是0就好了</strong>，是<strong>0的话索引没变</strong>，是<strong>1的话索引变成“原索引+oldCap”</strong>，可以看看下图为16扩充为32的resize示意图：</p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/544caeb82a329fa49cc99842818ed1ba_r.jpg" alt="JDK1.8扩容"></p>
<h6 id="HashMap-put-方法过程"><a href="#HashMap-put-方法过程" class="headerlink" title="HashMap.put()方法过程"></a>HashMap.put()方法过程</h6><p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/58e67eae921e4b431782c07444af824e_r.jpg" alt="img"></p>
<h6 id="小结"><a href="#小结" class="headerlink" title="小结"></a>小结</h6><p>(1) 扩容是一个特别耗性能的操作，所以当程序员在使用HashMap的时候，估算map的大小，初始化的时候给一个大致的数值，避免map进行频繁的扩容。</p>
<p>(2) 负载因子是可以修改的，也可以大于1，但是建议不要轻易修改，除非情况非常特殊。</p>
<p>(3) HashMap是线程不安全的，不要在并发的环境中同时操作HashMap，建议使用ConcurrentHashMap。</p>
<p>(4) JDK1.8引入红黑树大程度优化了HashMap的性能。</p>
<p>(5) 还没升级JDK1.8的，现在开始升级吧。HashMap的性能提升仅仅是JDK1.8的冰山一角。</p>
<h4 id="线程安全的ConcurrentHashMap"><a href="#线程安全的ConcurrentHashMap" class="headerlink" title="线程安全的ConcurrentHashMap"></a>线程安全的ConcurrentHashMap</h4><blockquote>
<p><a target="_blank" rel="noopener" href="https://mp.weixin.qq.com/s/AHWzboztt53ZfFZmsSnMSw">还不懂 ConcurrentHashMap ？这份源码分析了解一下 (qq.com)</a></p>
<p><a target="_blank" rel="noopener" href="https://blog.csdn.net/weixin_44460333/article/details/86770169">HashMap？ConcurrentHashMap？相信看完这篇没人能难住你！_Java团长的博客-CSDN博客</a></p>
</blockquote>
<p>前面提过的Hashtable和这里的ConcurrentHashMap，都是线程安全的。</p>
<p>怎么保证线程安全呢，答案就是==加锁==。</p>
<p>Hashtable中，直接使用了<code>synchronized</code>关键字来对<code>put()</code>和<code>get()</code>方法加锁。但是在竞争强烈的情况下，其效率非常低下。</p>
<p>而ConcurrenHashMap中，使用了一个“分段锁”的东西。</p>
<p>简单来说，JDK1.7使用Segment分段锁，结构是segemtn数组加HashEntiy；JDK8采用 <code>synchronized</code> 加 CAS锁的方式，结构是Node数组加链表/红黑树。 </p>
<h5 id="JDK1-7中的ConcurrentHashMap"><a href="#JDK1-7中的ConcurrentHashMap" class="headerlink" title="JDK1.7中的ConcurrentHashMap"></a>JDK1.7中的ConcurrentHashMap</h5><p><img src="https://ss.csdn.net/p?https://mmbiz.qpic.cn/mmbiz_png/QCu849YTaIPf1sDCN5zcDdGsibZwyzy9rmnTSzibQ6VEBXUhicBWHFae47ShkNzCRB7SZibuUN6gDmGkfeB5saAMQQ/640?wx_fmt=png" alt="JDK1.7的ConcurrentHashMap结构图"></p>
<p>如图所示，<strong>是由 Segment 数组、HashEntry 组成</strong>，和 HashMap 一样，仍然是数组加链表。</p>
<p>ConcurrentHashMap的核心成员变量，主要就是这里的segments数组：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">/**
* Segment 数组，存放数据时首先需要定位到具体的 Segment 中。
*/</span>
<span class="token keyword">final</span> <span class="token class-name">Segment</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> segments<span class="token punctuation">;</span>

<span class="token keyword">transient</span> <span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">></span></span> keySet<span class="token punctuation">;</span>
<span class="token keyword">transient</span> <span class="token class-name">Set</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">Map<span class="token punctuation">.</span>Entry</span><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span><span class="token punctuation">></span></span> entrySet<span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>然后去看Segment对象，ConcurrentMap的内部类，主要是table数组：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">class</span> <span class="token class-name">Segment</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span> <span class="token keyword">extends</span> <span class="token class-name">ReentrantLock</span> <span class="token keyword">implements</span> <span class="token class-name">Serializable</span> <span class="token punctuation">&#123;</span>

<span class="token keyword">private</span> <span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">long</span> serialVersionUID <span class="token operator">=</span> <span class="token number">2249069246763182397L</span><span class="token punctuation">;</span>

<span class="token comment">// 和 HashMap 中的 HashEntry 作用一样，真正存放数据的桶</span>
 <span class="token keyword">transient</span> <span class="token keyword">volatile</span> <span class="token class-name">HashEntry</span><span class="token generics"><span class="token punctuation">&lt;</span><span class="token class-name">K</span><span class="token punctuation">,</span><span class="token class-name">V</span><span class="token punctuation">></span></span><span class="token punctuation">[</span><span class="token punctuation">]</span> table<span class="token punctuation">;</span>

<span class="token keyword">transient</span> <span class="token keyword">int</span> count<span class="token punctuation">;</span>
<span class="token keyword">transient</span> <span class="token keyword">int</span> modCount<span class="token punctuation">;</span>
<span class="token keyword">transient</span> <span class="token keyword">int</span> threshold<span class="token punctuation">;</span>
<span class="token keyword">final</span> <span class="token keyword">float</span> loadFactor<span class="token punctuation">;</span>

<span class="token punctuation">&#125;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p>然后看一下HashEntry的组成：</p>
<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/image-20210618110453148.png" alt="image-20210618110453148"></p>
<p>HashEntry跟HashMap差不多的，但是不同点是，他使用 <strong>volatile</strong> 去修饰了他的数据 <strong>Value 还有下一个节点 next</strong>，保证了获取时的可见性。</p>
<h6 id="构造函数"><a href="#构造函数" class="headerlink" title="构造函数"></a>构造函数</h6><p>分为无参构造和有参构造，主要设置以下三个参数：</p>
<pre class="line-numbers language-java" data-language="java"><code class="language-java"><span class="token comment">/**
 * 默认初始化容量
 */</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> DEFAULT_INITIAL_CAPACITY <span class="token operator">=</span> <span class="token number">16</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * 默认负载因子
 */</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">float</span> DEFAULT_LOAD_FACTOR <span class="token operator">=</span> <span class="token number">0.75f</span><span class="token punctuation">;</span>

<span class="token comment">/**
 * 默认并发级别
 */</span>
<span class="token keyword">static</span> <span class="token keyword">final</span> <span class="token keyword">int</span> DEFAULT_CONCURRENCY_LEVEL <span class="token operator">=</span> <span class="token number">16</span><span class="token punctuation">;</span><span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<h6 id="put方法"><a href="#put方法" class="headerlink" title="put方法"></a>put方法</h6><p>尝试获取锁，如果获取失败就表明有其他线程在竞争，利用自旋获取锁。自旋达到一定次数则阻塞。</p>
<h6 id="get方法"><a href="#get方法" class="headerlink" title="get方法"></a>get方法</h6><p>直接将 <code>key</code> 进行hash之后，定位到Segment上，再Hash一次定位到具体元素上。</p>
<p>因为 value 是通过 <code>volatile</code> 修饰的，保证了内存的可见性，所以说每次获取都是新值。</p>
<h5 id="JDK1-8中的ConcurrentHashMap"><a href="#JDK1-8中的ConcurrentHashMap" class="headerlink" title="JDK1.8中的ConcurrentHashMap"></a>JDK1.8中的ConcurrentHashMap</h5><p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/java8_concurrenthashmap.png" alt="Java8 ConcurrentHashMap 存储结构（图片来自 javadoop）"></p>
<h6 id="put方法-1"><a href="#put方法-1" class="headerlink" title="put方法"></a>put方法</h6><ol>
<li><strong>Hash</strong>：根据 key 计算出 hashcode 。</li>
<li>判断是否需要进行<strong>初始化</strong></li>
<li>即为当前 key 定位出的 Node，如果为空表示当前位置可以写入数据，利用 CAS 尝试写入，失败则自旋保证成功。</li>
<li>如果当前位置的 hashcode == MOVED == -1,则需要进行扩容。</li>
<li>如果都不满足，则利用 synchronized 锁写入数据。如果数量大于 TREEIFY_THRESHOLD 则要转换为红黑树。</li>
</ol>
<pre class="line-numbers language-java" data-language="java"><code class="language-java">hashcode <span class="token operator">=</span> <span class="token class-name">Hash</span><span class="token punctuation">(</span><span class="token class-name">Key</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>遍历<span class="token class-name">Node</span>数组<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token class-name">Node</span>数组为空<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        初始化<span class="token punctuation">(</span><span class="token class-name">Node</span>数组<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
    <span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span> key 对应的 <span class="token class-name">Node</span><span class="token punctuation">[</span>hashcode<span class="token punctuation">]</span> 的 value 为空<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span> CAS写入<span class="token punctuation">(</span><span class="token class-name">Node</span><span class="token punctuation">[</span>hashcode<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">)</span> <span class="token keyword">break</span><span class="token punctuation">;</span>
        <span class="token keyword">else</span> 自旋；
    <span class="token punctuation">&#125;</span>
    <span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span> <span class="token class-name">Node</span><span class="token punctuation">[</span>hashcode<span class="token punctuation">]</span> 不存在<span class="token punctuation">)</span><span class="token punctuation">&#123;</span>
    	扩容；
    <span class="token punctuation">&#125;</span>
    <span class="token keyword">else</span><span class="token punctuation">&#123;</span>
        <span class="token keyword">synchronized</span>锁写入<span class="token punctuation">;</span>
        <span class="token keyword">if</span><span class="token punctuation">(</span>判断链表长度<span class="token punctuation">)</span>
            是否要转换为红黑树<span class="token punctuation">;</span>
    <span class="token punctuation">&#125;</span>
<span class="token punctuation">&#125;</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>

<p><img src="https://gitee.com/songx86/SongPicBed/raw/master/img/20210304170631489.png" alt="JDK1.8的ConcurrentMap.put()"></p>

                
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